a(13+6a)=28

Solution for a(13+6a)=28 equation:

a(13+6a)=28

We move all terms to the left:

a(13+6a)-(28)=0

We add all the numbers together, and all the variables

a(6a+13)-28=0

We multiply parentheses

6a^2+13a-28=0

a = 6; b = 13; c = -28;
Δ = b2-4ac
Δ = 132-4·6·(-28)
Δ = 841
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{841}=29$
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(13)-29}{2*6}=\frac{-42}{12}
=-3+1/2
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(13)+29}{2*6}=\frac{16}{12}
=1+1/3
$